Methods and apparatus for treatment of respiratory disorders

ABSTRACT

Disclosed are methods, apparatus and systems for treating a respiratory disorder in a patient. The apparatus comprises a pressure generator configured to generate a flow of air so as to provide ventilatory support to the patient; a transducer configured to generate a flow signal representing a property of the flow of air; and a controller configured to analyse the flow signal to estimate the inspiratory volume and the expiratory volume of a breath of the patient and servo-control the degree of ventilatory support to adjust an estimated tidal volume toward a target tidal volume. A gain of the servo-control is dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume. The method comprises operating an apparatus or system in a similar manner.

1 CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/AU2018/050441 filed May 11, 2018, published in English, which claims priority from Australian Provisional Patent Application No. 2017901773 filed May 12, 2017, all of which are incorporated herein by reference.

2 BACKGROUND OF THE TECHNOLOGY 2.1 Field of the Technology

The present technology relates to one or more of the screening, diagnosis, monitoring, treatment, prevention and amelioration of respiratory-related disorders. The present technology also relates to medical devices or apparatus, and their operation and use.

2.2 Description of the Related Art

2.2.1 Human Respiratory System and its Disorders

The respiratory system of the body facilitates gas exchange. The nose and mouth form the entrance to the airways of a patient.

The airways include a series of branching tubes, which become narrower, shorter and more numerous as they penetrate deeper into the lung. The prime function of the lung is gas exchange, allowing oxygen to move from the inhaled air into the venous blood and carbon dioxide to move in the opposite direction. The trachea divides into right and left main bronchi, which further divide eventually into terminal bronchioles. The bronchi make up the conducting airways, and do not take part in gas exchange. Further divisions of the airways lead to the respiratory bronchioles, and eventually to the alveoli. The alveolated region of the lung is where the gas exchange takes place, and is referred to as the respiratory zone. See “Respiratory Physiology”, by John B. West, Lippincott Williams & Wilkins, 9th edition published 2012.

A range of respiratory disorders exist. Certain disorders may be characterised by particular events, e.g. apneas, hypopneas, and hyperpneas.

Examples of respiratory disorders include Obstructive Sleep Apnea (OSA), Cheyne-Stokes Respiration (CSR), respiratory insufficiency, Obesity Hyperventilation Syndrome (OHS), Chronic Obstructive Pulmonary Disease (COPD), Neuromuscular Disease (NMD) and Chest wall disorders.

Obstructive Sleep Apnea (OSA), a form of Sleep Disordered Breathing (SDB), is characterised by events including occlusion or obstruction of the upper air passage during sleep. It results from a combination of an abnormally small upper airway and the normal loss of muscle tone in the region of the tongue, soft palate and posterior oropharyngeal wall during sleep. The condition causes the affected patient to stop breathing for periods typically of 30 to 120 seconds in duration, sometimes 200 to 300 times per night. It often causes excessive daytime somnolence, and it may cause cardiovascular disease and brain damage. The syndrome is a common disorder, particularly in middle aged overweight males, although a person affected may have no awareness of the problem. See U.S. Pat. No. 4,944,310 (Sullivan).

Cheyne-Stokes Respiration (CSR) is another form of sleep disordered breathing. CSR is a disorder of a patient's respiratory controller in which there are rhythmic alternating periods of waxing and waning ventilation known as CSR cycles. CSR is characterised by repetitive de-oxygenation and re-oxygenation of the arterial blood. It is possible that CSR is harmful because of the repetitive hypoxia. In some patients CSR is associated with repetitive arousal from sleep, which causes severe sleep disruption, increased sympathetic activity, and increased afterload. See U.S. Pat. No. 6,532,959 (Berthon-Jones).

Respiratory failure is an umbrella term for respiratory disorders in which the lungs are unable to inspire sufficient oxygen or exhale sufficient CO₂ to meet the patient's needs. Respiratory failure may encompass some or all of the following disorders.

A patient with respiratory insufficiency (a form of respiratory failure) may experience abnormal shortness of breath on exercise.

Obesity Hyperventilation Syndrome (OHS) is defined as the combination of severe obesity and awake chronic hypercapnia, in the absence of other known causes for hypoventilation. Symptoms include dyspnea, morning headache and excessive daytime sleepiness.

Chronic Obstructive Pulmonary Disease (COPD) encompasses any of a group of lower airway diseases that have certain characteristics in common. These include increased resistance to air movement, extended expiratory phase of respiration, and loss of the normal elasticity of the lung. Examples of COPD are emphysema and chronic bronchitis. COPD is caused by chronic tobacco smoking (primary risk factor), occupational exposures, air pollution and genetic factors. Symptoms include: dyspnea on exertion, chronic cough and sputum production.

Neuromuscular Disease (NMD) is a broad term that encompasses many diseases and ailments that impair the functioning of the muscles either directly via intrinsic muscle pathology, or indirectly via nerve pathology. Some NMD patients are characterised by progressive muscular impairment leading to loss of ambulation, being wheelchair-bound, swallowing difficulties, respiratory muscle weakness and, eventually, death from respiratory failure. Neuromuscular disorders can be divided into rapidly progressive and slowly progressive: (i) Rapidly progressive disorders: Characterised by muscle impairment that worsens over months and results in death within a few years (e.g. Amyotrophic lateral sclerosis (ALS) and Duchenne muscular dystrophy (DMD) in teenagers); (ii) Variable or slowly progressive disorders: Characterised by muscle impairment that worsens over years and only mildly reduces life expectancy (e.g. Limb girdle, Facioscapulohumeral and Myotonic muscular dystrophy). Symptoms of respiratory failure in NMD include: increasing generalised weakness, dysphagia, dyspnea on exertion and at rest, fatigue, sleepiness, morning headache, and difficulties with concentration and mood changes.

Chest wall disorders are a group of thoracic deformities that result in inefficient coupling between the respiratory muscles and the thoracic cage. The disorders are usually characterised by a restrictive defect and share the potential of long term hypercapnic respiratory failure. Scoliosis and/or kyphoscoliosis may cause severe respiratory failure. Symptoms of respiratory failure include: dyspnea on exertion, peripheral oedema, orthopnea, repeated chest infections, morning headaches, fatigue, poor sleep quality and loss of appetite.

A range of therapies have been used to treat or ameliorate such conditions. Furthermore, otherwise healthy individuals may take advantage of such therapies to prevent respiratory disorders from arising. However, these have a number of shortcomings.

2.2.2 Therapy

Various therapies, such as Continuous Positive Airway Pressure (CPAP) therapy, Non-invasive ventilation (NIV) and Invasive ventilation (IV) have been used to treat one or more of the above respiratory disorders.

Continuous Positive Airway Pressure (CPAP) therapy has been used to treat Obstructive Sleep Apnea (OSA). The mechanism of action is that continuous positive airway pressure acts as a pneumatic splint and may prevent upper airway occlusion, such as by pushing the soft palate and tongue forward and away from the posterior oropharyngeal wall. Treatment of OSA by CPAP therapy may be voluntary, and hence patients may elect not to comply with therapy if they find devices used to provide such therapy one or more of: uncomfortable, difficult to use, expensive and aesthetically unappealing.

Non-invasive ventilation (NIV) provides ventilatory support to a patient through the upper airways to assist the patient breathing and/or maintain adequate oxygen levels in the body by doing some or all of the work of breathing. The ventilatory support is provided via a non-invasive patient interface. NIV has been used to treat CSR and respiratory failure, in forms such as OHS, COPD, NMD and Chest Wall disorders.

Invasive ventilation (IV) provides ventilatory support to patients that are no longer able to effectively breathe themselves and may be provided using a tracheostomy tube.

In some forms, the comfort and effectiveness of these therapies may be improved.

2.2.3 Treatment Systems

These therapies may be provided by a treatment system or device. Such systems and devices may also be used to screen, diagnose, or monitor a condition without treating it.

A treatment system may comprise a Respiratory Pressure Therapy Device (RPT device), an air circuit, a humidifier, a patient interface, and data management.

2.2.3.1 Respiratory Pressure Therapy (RPT) Device

A respiratory pressure therapy (RPT) device may be used individually or as part of a system to deliver one or more of a number of therapies described above, such as by operating the device to generate a flow of air for delivery to an interface to the airways. The flow of air may be pressurised. RPT devices typically comprise a pressure generator, such as a motor-driven blower or a compressed gas reservoir, and are configured to supply a flow of air to the airway of a patient. In some cases, the flow of air may be supplied to the airway of the patient at positive pressure. The outlet of the RPT device is connected via an air circuit to a patient interface such as those described below.

One known RPT device used for treating sleep disordered breathing is the S9 Sleep Therapy System, manufactured by ResMed Limited. Another example of an RPT device is a ventilator. The ResMed Elisée™ 150 ventilator and ResMed VS III™ ventilator may provide support for invasive and non-invasive dependent ventilation suitable for adult or paediatric patients for treating a number of conditions. These ventilators provide volumetric and barometric ventilation modes with a single or double limb circuit.

2.2.3.2 Patient Interface

A patient interface may be used to interface respiratory equipment to its wearer, for example by providing a flow of air to an entrance to the airways. The flow of air may be provided via a mask to the nose and/or mouth, a tube to the mouth or a tracheostomy tube to the trachea of a patient. Depending upon the therapy to be applied, the patient interface may form a seal, e.g., with a region of the patient's face, to facilitate the delivery of gas at a pressure at sufficient variance with ambient pressure to effect therapy, e.g., at a positive pressure of about 10 cmH₂O relative to ambient pressure. For other forms of therapy, such as the delivery of oxygen, the patient interface may not include a seal sufficient to facilitate delivery to the airways of a supply of gas at a positive pressure of about 10 cmH₂O.

2.2.3.3 Humidifier

Delivery of a flow of air without humidification may cause drying of airways. The use of a humidifier with an RPT device and the patient interface produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. In addition in cooler climates, warm air applied generally to the face area in and about the patient interface is more comfortable than cold air.

2.2.3.4 Data Management

There may be clinical reasons to obtain data to determine whether the patient prescribed with respiratory therapy has been “compliant”, e.g. that the patient has used their RPT device according to one or more “compliance rules”. One example of a compliance rule for CPAP therapy is that a patient, in order to be deemed compliant, is required to use the RPT device for at least four hours a night for at least 21 of 30 consecutive days. In order to determine a patient's compliance, a provider of the RPT device, such as a health care provider, may manually obtain data describing the patient's therapy using the RPT device, calculate the usage over a predetermined time period, and compare with the compliance rule. Once the health care provider has determined that the patient has used their RPT device according to the compliance rule, the health care provider may notify a third party that the patient is compliant.

There may be other aspects of a patient's therapy that would benefit from communication of therapy data to a third party or external system.

2.2.3.5 Vent Technologies

Some forms of treatment systems may include a vent to allow the washout of exhaled carbon dioxide. The vent may allow a flow of gas from an interior space of a patient interface, e.g., the plenum chamber, to an exterior of the patient interface, e.g., to ambient.

3 BRIEF SUMMARY OF THE TECHNOLOGY

The present technology is directed towards providing medical devices used in the screening, diagnosis, monitoring, amelioration, treatment, or prevention of respiratory disorders having one or more of improved comfort, cost, efficacy, ease of use and manufacturability.

A first aspect of the present technology relates to apparatus used in the screening, diagnosis, monitoring, amelioration, treatment or prevention of a respiratory disorder.

Another aspect of the present technology relates to methods used in the screening, diagnosis, monitoring, amelioration, treatment or prevention of a respiratory disorder.

One form of the present technology comprises apparatus for treatment of a respiratory disorder using a safety volume servo-ventilation mode, in which the gain of the servo-control of pressure support is based on a differential between estimated inspiratory and expiratory volumes.

According to one aspect of the present technology, there is provided apparatus for treating a respiratory disorder in a patient. The apparatus comprises a pressure generator configured to generate a flow of air so as to provide ventilatory support to the patient; a transducer configured to generate a signal representing a property of the flow of air; and a controller configured to analyse the signal to estimate an inspiratory volume and an expiratory volume of a breath of the patient and servo-control the degree of ventilatory support to adjust an estimated tidal volume toward a target tidal volume. A gain of the servo-control is dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume.

In an example of this aspect, the gain decreases as the difference between the estimated inspiratory volume and the estimated expiratory volume increases. In a further example, the gain is dependent on the difference between the estimated inspiratory volume and the estimated expiratory volume relative to the estimated tidal volume. Further still, the gain is dependent on the absolute magnitude of the difference between the estimated inspiratory volume and the estimated expiratory volume.

In another example of this aspect, the degree of the ventilatory support is a pressure support of the ventilatory support.

According to a further aspect of the present technology, there is provided a method of operating a respiratory treatment apparatus configured to generate a flow of air so as to provide ventilatory support to a patient. The method comprises measuring a property of the flow of air, using a transducer; analysing, in a controller, the measured property to estimate the inspiratory volume and the expiratory volume of a breath of the patient; calculating, in a controller, a gain dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume; and servo-controlling, by a controller, the respiratory treatment apparatus using the calculated gain to adjust an estimated tidal volume for the patient toward a target tidal volume.

In an example of this aspect, the gain is dependent on the difference between the estimated inspiratory volume and the estimated expiratory volume relative to the estimated tidal volume. In other examples, the gain is dependent on the absolute magnitude of the difference between the estimated inspiratory volume and the estimated expiratory volume. In other examples, the degree of the ventilatory support is a pressure support of the ventilatory support.

According to a further aspect of the present technology, there is provided a system for treating a respiratory disorder in a patient. The system comprises means for generating a flow of air so as to provide ventilatory support to the patient; means for generating a signal representing a property of the flow of air; means for analysing the signal to estimate the inspiratory volume and the expiratory volume of a breath of the patient; and means for servo-controlling the degree of ventilatory support to adjust an estimated tidal volume toward a target tidal volume. A gain of the servo-control is dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume.

According to a further aspect of the present technology, an apparatus for treating a respiratory disorder in a patient is provided. The apparatus comprises a blower configured to deliver a supply of air that provides ventilatory support to the patient; a transducer configured to generate a signal representing a property of the supply of air; and a controller. The controller is configured to analyse the signal to estimate an inspiratory volume and an expiratory volume of a breath of the patient; and adjust servo-control gain based on a difference between the estimated inspiratory volume and the estimated expiratory volume.

In an example of this aspect, the controller adjusts the servo-control gain so that the gain decreases as the difference between the estimated inspiratory volume and the estimated expiratory volume increases. In another example, an adjustment in servo-control gain causes a reduction in a rate of adjustment of pressure support of the supply of air. In other examples, the controller is configured to adjust the servo-control gain when the difference is greater than or equal to 20%, or further configured to adjust the servo-control gain to a default value when the difference returns to a value that is less than 20%.

The methods, systems, devices and apparatus described may be implemented so as to improve the functionality of a processor, such as a processor of a specific purpose computer, respiratory monitor and/or a respiratory therapy apparatus. Moreover, the described methods, systems, devices and apparatus can provide improvements in the technological field of automated management, monitoring and/or treatment of respiratory conditions, including, for example, sleep disordered breathing.

Of course, portions of the aspects may form sub-aspects of the present technology. Also, various ones of the sub-aspects, aspects and/or example may be combined in various manners and also constitute additional aspects or sub-aspects of the present technology.

Other features of the technology will be apparent from consideration of the information contained in the following detailed description, abstract, drawings and claims.

4 BRIEF DESCRIPTION OF THE DRAWINGS

The present technology is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements including:

4.1 Treatment Systems

FIG. 1 shows a system including a patient 1000 wearing a patient interface 3000, in the form of a full-face mask, receiving a supply of air at positive pressure from an RPT device 4000. Air from the RPT device is humidified in a humidifier 5000, and passes along an air circuit 4170 to the patient 1000. The patient is sleeping in a side sleeping position.

4.2 Respiratory System and Facial Anatomy

FIG. 2 shows an overview of a human respiratory system including the nasal and oral cavities, the larynx, vocal folds, oesophagus, trachea, bronchus, lung, alveolar sacs, heart and diaphragm.

4.3 Patient Interface

FIG. 3 shows a patient interface in the form of a nasal mask in accordance with one form of the present technology.

4.4 RPT Device

FIG. 4A shows an RPT device in accordance with one form of the present technology.

FIG. 4B is a schematic diagram of the pneumatic path of an RPT device in accordance with one form of the present technology. The directions of upstream and downstream are indicated with reference to the blower and the patient interface. The blower is defined to be upstream of the patient interface and the patient interface is defined to be downstream of the blower, regardless of the actual flow direction at any particular moment. Items which are located within the pneumatic path between the blower and the patient interface are downstream of the blower and upstream of the patient interface.

FIG. 4C is a schematic diagram of the electrical components of an RPT device in accordance with one form of the present technology.

FIG. 4D is a schematic diagram of the algorithms implemented in an RPT device in accordance with one form of the present technology.

4.5 Humidifier

FIG. 5A shows an isometric view of a humidifier in accordance with one form of the present technology.

FIG. 5B shows an isometric view of a humidifier in accordance with one form of the present technology, showing a humidifier reservoir 5110 removed from the humidifier reservoir dock 5130.

4.6 Breathing Waveforms

FIG. 6 shows a model typical breath waveform of a person while sleeping.

4.7 Respiratory Pressure Therapy Modes

FIG. 7A is a graph illustrating undesirable behaviour of pressure support during sudden leak changes in conventional safety volume mode.

FIG. 7B is a graph illustrating behaviour of pressure support during sudden leak changes in safety volume mode according to one form of the present technology.

FIG. 8 is a graph illustrating of the adjustment of the servo-control gain in safety volume mode as a function of a relative differential between inspiratory and expiratory volumes.

FIG. 9 is a flow chart that provides an overview of method for adjusting servo-controller gain in a respiratory apparatus.

5 DETAILED DESCRIPTION OF EXAMPLES OF THE TECHNOLOGY

Before the present technology is described in further detail, it is to be understood that the technology is not limited to the particular examples described herein, which may vary. It is also to be understood that the terminology used in this disclosure is for the purpose of describing only the particular examples discussed herein, and is not intended to be limiting.

The following description is provided in relation to various examples which may share one or more common characteristics and/or features. It is to be understood that one or more features of any one example may be combinable with one or more features of another example or other examples. In addition, any single feature or combination of features in any of the examples may constitute a further example.

5.1 Therapy

In one form, the present technology comprises a method for treating a respiratory disorder comprising the step of applying positive pressure to the entrance of the airways of a patient 1000.

In certain examples of the present technology, a supply of air at positive pressure is provided to the nasal passages of the patient via one or both nares.

In certain examples of the present technology, mouth breathing is limited, restricted or prevented.

5.2 Treatment Systems

In one form, the present technology comprises an apparatus or device for treating a respiratory disorder. The apparatus or device may comprise an RPT device 4000 for supplying pressurised air to the patient 1000 via an air circuit 4170 to a patient interface 3000, as shown for example in FIG. 1.

5.3 Patient Interface

A non-invasive patient interface 3000 in accordance with one aspect of the present technology comprises the following functional aspects: a seal-forming structure 3100, a plenum chamber 3200, a positioning and stabilising structure 3300, a vent 3400, one form of connection port 3600 for connection to air circuit 4170, and a forehead support 3700, such as depicted for example in FIG. 3. In some forms a functional aspect may be provided by one or more physical components. In some forms, one physical component may provide one or more functional aspects. In use the seal-forming structure 3100 is arranged to surround an entrance to the airways of the patient so as to facilitate the supply of air at positive pressure to the airways.

5.4 RPT Device

An RPT device 4000 in accordance with one aspect of the present technology comprises mechanical, pneumatic, and/or electrical components and is configured to execute one or more algorithms 4300, such as any of the methods, in whole or in part, described herein. The RPT device 4000 may be configured to generate a flow of air for delivery to a patient's airways, such as to treat one or more of the respiratory conditions described elsewhere in the present document. FIGS. 4A through 4D provide illustrative examples of the components or schematics that may comprise RPT device 4000.

The RPT device may have an external housing 4010, formed in two parts, an upper portion 4012 and a lower portion 4014. Furthermore, the external housing 4010 may include one or more panel(s) 4015. The RPT device 4000 comprises a chassis 4016 that supports one or more internal components of the RPT device 4000. The RPT device 4000 may include a handle 4018.

The pneumatic path of the RPT device 4000 may comprise one or more air path items, e.g., an inlet air filter 4112, an inlet muffler 4122, a pressure generator 4140 capable of supplying air at positive pressure (e.g., a blower 4142), an outlet muffler 4124 and one or more transducers 4270, such as pressure sensors 4272 and flow rate sensors 4274.

One or more of the air path items may be located within a removable unitary structure which will be referred to as a pneumatic block 4020. The pneumatic block 4020 may be located within the external housing 4010. In one form a pneumatic block 4020 is supported by, or formed as part of the chassis 4016.

The RPT device 4000 may have an electrical power supply 4210, one or more input devices 4220, a central controller 4230, a therapy device controller 4240, a pressure generator 4140, one or more protection circuits 4250, memory 4260, transducers 4270, data communication interface 4280 and one or more output devices 4290. Electrical components 4200 may be mounted on a single Printed Circuit Board Assembly (PCBA) 4202. In an alternative form, the RPT device 4000 may include more than one PCBA 4202.

5.4.1 RPT Device Mechanical & Pneumatic Components

An RPT device may comprise one or more of the following components in an integral unit. In an alternative form, one or more of the following components may be located as respective separate units.

5.4.1.1 Air Filter(s)

An RPT device in accordance with one form of the present technology may include an air filter 4110, or a plurality of air filters 4110.

In one form, an inlet air filter 4112 is located at the beginning of the pneumatic path upstream of a pressure generator 4140.

In one form, an outlet air filter 4114, for example an antibacterial filter, is located between an outlet of the pneumatic block 4020 and a patient interface 3000.

5.4.1.2 Muffler(s)

An RPT device in accordance with one form of the present technology may include a muffler 4120, or a plurality of mufflers 4120.

In one form of the present technology, an inlet muffler 4122 is located in the pneumatic path upstream of a pressure generator 4140.

In one form of the present technology, an outlet muffler 4124 is located in the pneumatic path between the pressure generator 4140 and a patient interface 3000.

5.4.1.3 Pressure Generator

In one form of the present technology, a pressure generator 4140 for producing a flow, or a supply, of air at positive pressure is a controllable blower 4142. For example the blower 4142 may include a brushless DC motor 4144 with one or more impellers housed in a blower housing, such as in a volute. The blower may be capable of delivering a supply of air, for example at a rate of up to about 120 litres/minute, at a positive pressure in a range from about 4 cmH₂O to about 20 cmH₂O, or in other forms up to about 30 cmH₂O. The blower may be as described in any one of the following patents or patent applications the contents of which are incorporated herein by reference in their entirety: U.S. Pat. Nos. 7,866,944; 8,638,014; 8,636,479; and PCT Patent Application Publication No. WO 2013/020167.

The pressure generator 4140 is under the control of the therapy device controller 4240.

In other forms, a pressure generator 4140 may be a piston-driven pump, a pressure regulator connected to a high pressure source (e.g. compressed air reservoir), or a bellows.

5.4.1.4 Transducer(s)

Transducers may be internal of the RPT device, or external of the RPT device. External transducers may be located for example on or form part of the air circuit, e.g., the patient interface. External transducers may be in the form of non-contact sensors such as a Doppler radar movement sensor that transmit or transfer data to the RPT device.

In one form of the present technology, one or more transducers 4270 are located upstream and/or downstream of the pressure generator 4140. The one or more transducers 4270 may be constructed and arranged to generate signals representing properties of the flow of air such as a flow rate, a pressure or a temperature at that point in the pneumatic path.

In one form of the present technology, one or more transducers 4270 may be located proximate to the patient interface 3000.

In one form, a signal from a transducer 4270 may be filtered, such as by low-pass, high-pass or band-pass filtering.

5.4.1.4.1 Flow Rate Sensor

A flow rate sensor 4274 in accordance with the present technology may be based on a differential pressure transducer, for example, an SDP600 Series differential pressure transducer from SENSIRION.

In one form, a signal representing a flow rate from the flow rate sensor 4274 is received by the central controller 4230.

5.4.1.4.2 Pressure Sensor

A pressure sensor 4272 in accordance with the present technology is located in fluid communication with the pneumatic path. An example of a suitable pressure sensor is a transducer from the HONEYWELL ASDX series. An alternative suitable pressure sensor is a transducer from the NPA Series from GENERAL ELECTRIC.

In one form, a signal from the pressure sensor 4272 is received by the central controller 4230.

5.4.1.4.3 Motor Speed Transducer

In one form of the present technology a motor speed transducer 4276 is used to determine a rotational velocity of the motor 4144 and/or the blower 4142. A motor speed signal from the motor speed transducer 4276 may be provided to the therapy device controller 4240. The motor speed transducer 4276 may, for example, be a speed sensor, such as a Hall effect sensor.

5.4.1.5 Anti-Spill Back Valve

In one form of the present technology, an anti-spill back valve 4160 is located between the humidifier 5000 and the pneumatic block 4020. The anti-spill back valve is constructed and arranged to reduce the risk that water will flow upstream from the humidifier 5000, for example to the motor 4144.

5.4.2 RPT Device Electrical Components

5.4.2.1 Power Supply

A power supply 4210 may be located internal or external of the external housing 4010 of the RPT device 4000.

In one form of the present technology, power supply 4210 provides electrical power to the RPT device 4000 only. In another form of the present technology, power supply 4210 provides electrical power to both RPT device 4000 and humidifier 5000.

5.4.2.2 Input Devices

In one form of the present technology, an RPT device 4000 includes one or more input devices 4220 in the form of buttons, switches or dials to allow a person to interact with the device. The buttons, switches or dials may be physical devices, or software devices accessible via a touch screen. The buttons, switches or dials may, in one form, be physically connected to the external housing 4010, or may, in another form, be in wireless communication with a receiver that is in electrical connection to the central controller 4230.

In one form, the input device 4220 may be constructed and arranged to allow a person to select a value and/or a menu option.

5.4.2.3 Central Controller

In one form of the present technology, the central controller 4230 is one or a plurality of processors suitable to control an RPT device 4000.

Suitable processors may include an x86 INTEL processor, a processor based on ARM® Cortex®-M processor from ARM Holdings such as an STM32 series microcontroller from ST MICROELECTRONIC. In certain alternative forms of the present technology, a 32-bit RISC CPU, such as an STR9 series microcontroller from ST MICROELECTRONICS or a 16-bit RISC CPU such as a processor from the MSP430 family of microcontrollers, manufactured by TEXAS INSTRUMENTS may also be suitable.

In one form of the present technology, the central controller 4230 is a dedicated electronic circuit.

In one form, the central controller 4230 is an application-specific integrated circuit. In another form, the central controller 4230 comprises discrete electronic components.

The central controller 4230 may be configured to receive input signal(s) from one or more transducers 4270, one or more input devices 4220, and the humidifier 5000.

The central controller 4230 may be configured to provide output signal(s) to one or more of an output device 4290, a therapy device controller 4240, a data communication interface 4280, and the humidifier 5000.

In some forms of the present technology, the central controller 4230 is configured to implement the one or more methodologies described herein, such as the one or more algorithms 4300 expressed as computer programs stored in a non-transitory computer readable storage medium, such as memory 4260. In some forms of the present technology, the central controller 4230 may be integrated with an RPT device 4000. However, in some forms of the present technology, some methodologies may be performed by a remotely located device. For example, the remotely located device may determine control settings for a ventilator or detect respiratory related events by analysis of stored data such as from any of the sensors described herein.

5.4.2.4 Clock

The RPT device 4000 may include a clock 4232 that is connected to the central controller 4230.

5.4.2.5 Therapy Device Controller

In one form of the present technology, therapy device controller 4240 is a therapy control module 4330 that forms part of the algorithms 4300 executed by the central controller 4230.

In one form of the present technology, therapy device controller 4240 is a dedicated motor control integrated circuit. For example, in one form a MC33035 brushless DC motor controller, manufactured by ONSEMI is used.

5.4.2.6 Protection Circuits

The one or more protection circuits 4250 in accordance with the present technology may comprise an electrical protection circuit, a temperature and/or pressure safety circuit.

5.4.2.7 Memory

In accordance with one form of the present technology the RPT device 4000 includes memory 4260, e.g., non-volatile memory. In some forms, memory 4260 may include battery powered static RAM. In some forms, memory 4260 may include volatile RAM.

Memory 4260 may be located on the PCBA 4202. Memory 4260 may be in the form of EEPROM, or NAND flash.

Additionally or alternatively, RPT device 4000 includes a removable form of memory 4260, for example a memory card made in accordance with the Secure Digital (SD) standard.

In one form of the present technology, the memory 4260 acts as a non-transitory computer readable storage medium on which is stored computer program instructions expressing the one or more methodologies described herein, such as the one or more algorithms 4300.

5.4.2.8 Data Communication Systems

In one form of the present technology, a data communication interface 4280 is provided, and is connected to the central controller 4230. Data communication interface 4280 may be connectable to a remote external communication network 4282 and/or a local external communication network 4284. The remote external communication network 4282 may be connectable to a remote external device 4286. The local external communication network 4284 may be connectable to a local external device 4288.

In one form, data communication interface 4280 is part of the central controller 4230. In another form, data communication interface 4280 is separate from the central controller 4230, and may comprise an integrated circuit or a processor.

In one form, remote external communication network 4282 is the Internet. The data communication interface 4280 may use wired communication (e.g. via Ethernet, or optical fibre) or a wireless protocol (e.g. CDMA, GSM, LTE) to connect to the Internet.

In one form, local external communication network 4284 utilises one or more communication standards, such as Bluetooth, or a consumer infrared protocol.

In one form, remote external device 4286 is one or more computers, for example a cluster of networked computers. In one form, remote external device 4286 may be virtual computers, rather than physical computers. In either case, such a remote external device 4286 may be accessible to an appropriately authorised person such as a clinician.

The local external device 4288 may be a personal computer, mobile phone, tablet or remote control.

5.4.2.9 Output Devices Including Optional Display, Alarms

An output device 4290 in accordance with the present technology may take the form of one or more of a visual, audio and haptic unit. A visual display may be a Liquid Crystal Display (LCD) or Light Emitting Diode (LED) display.

5.4.2.9.1 Display Driver

A display driver 4292 receives as an input the characters, symbols, or images intended for display on the display 4294, and converts them to commands that cause the display 4294 to display those characters, symbols, or images.

5.4.2.9.2 Display

A display 4294 is configured to visually display characters, symbols, or images in response to commands received from the display driver 4292. For example, the display 4294 may be an eight-segment display, in which case the display driver 4292 converts each character or symbol, such as the figure “0”, to eight logical signals indicating whether the eight respective segments are to be activated to display a particular character or symbol.

5.4.3 RPT Device Algorithms

As mentioned above, in some forms of the present technology, the central controller 4230 may be configured to implement one or more algorithms 4300 expressed as computer programs stored in a non-transitory computer readable storage medium, such as memory 4260. The algorithms 4300 are generally grouped into groups referred to as modules.

5.4.3.1 Pre-Processing Module

A pre-processing module 4310 in accordance with one form of the present technology receives as an input a signal from a transducer 4270, for example a flow rate sensor 4274 or pressure sensor 4272, and performs one or more process steps to calculate one or more output values that will be used as an input to another module, for example a therapy engine module 4320, and may be depicted as shown for example in FIG. 4D.

In one form of the present technology, the output values include the interface or mask pressure Pm, the respiratory flow rate Qr, and the leak flow rate Ql.

In various forms of the present technology, the pre-processing module 4310 comprises one or more of the following algorithms: pressure compensation 4312, vent flow rate estimation 4314, leak flow rate estimation 4316, and respiratory flow rate estimation 4318.

5.4.3.1.1 Pressure Compensation

In one form of the present technology, a pressure compensation algorithm 4312 receives as an input a signal indicative of the pressure in the pneumatic path proximal to an outlet of the pneumatic block. The pressure compensation algorithm 4312 estimates the pressure drop through the air circuit 4170 and provides as an output an estimated pressure, Pm, in the patient interface 3000.

5.4.3.1.2 Vent Flow Rate Estimation

In one form of the present technology, a vent flow rate estimation algorithm 4314 receives as an input an estimated pressure, Pm, in the patient interface 3000 and estimates a vent flow rate of air, Qv, from a vent 3400 in a patient interface 3000.

5.4.3.1.3 Leak Flow Rate Estimation

In one form of the present technology, a leak flow rate estimation algorithm 4316 receives as an input a total flow rate, Qt, and a vent flow rate Qv, and provides as an output an estimate of the leak flow rate Ql. In one form, the leak flow rate estimation algorithm estimates the leak flow rate Ql by calculating an average of the difference between total flow rate Qt and vent flow rate Qv over a period sufficiently long to include several breathing cycles, e.g. about 10 seconds.

In one form, the leak flow rate estimation algorithm 4316 receives as an input a total flow rate Qt, a vent flow rate Qv, and an estimated pressure, Pm, in the patient interface 3000, and provides as an output a leak flow rate Ql, by calculating a leak conductance, and determining a leak flow rate Ql to be a function of leak conductance and pressure, Pm. Leak conductance is calculated as the quotient of low pass filtered non-vent flow rate equal to the difference between total flow rate Qt and vent flow rate Qv, and low pass filtered square root of pressure Pm, where the low pass filter time constant has a value sufficiently long to include several breathing cycles, e.g. about 10 seconds. The leak flow rate Ql may be estimated as the product of leak conductance and a function of pressure, Pm.

A sudden change in leak is a change on a time scale that is shorter than the leak flow rate estimation algorithm 4316 can initially keep up with, i.e. of the order of a breathing cycle or less. In certain therapy modes such sudden changes in leak need special handling, as described below.

5.4.3.1.4 Respiratory Flow Rate Estimation

In one form of the present technology, a respiratory flow rate estimation algorithm 4318 receives as an input a total flow rate, Qt, a vent flow rate, Qv, and a leak flow rate, Ql, and estimates a respiratory flow rate of air, Qr, to the patient, by subtracting the vent flow rate Qv and the leak flow rate Ql from the total flow rate Qt.

5.4.3.2 Therapy Engine Module

In one form of the present technology, a therapy engine module 4320 receives as inputs one or more of a pressure, Pm, in a patient interface 3000, and a respiratory flow rate of air to a patient, Qr, and provides as an output one or more therapy parameters.

In one form of the present technology, a therapy parameter is a treatment pressure Pt.

In one form of the present technology, therapy parameters are one or more of an amplitude of a pressure variation, a base pressure, and a target tidal volume.

In various forms, the therapy engine module 4320 comprises one or more of the following algorithms: phase determination 4321, waveform determination 4322, tidal volume estimation 4323, and therapy parameter determination 4329.

5.4.3.2.1 Phase Determination

In one form of the present technology, the RPT device 4000 may determine breathing phase.

In one form of the present technology, a phase determination algorithm 4321 receives as an input a signal indicative of respiratory flow rate, Qr, and provides as an output a phase Φ of a current breathing cycle of a patient 1000.

In some forms, known as discrete phase determination, the phase output Φ is a discrete variable. One implementation of discrete phase determination provides a bi-valued phase output Φ with values of either inhalation or exhalation, for example represented as values of 0 and 0.5 revolutions respectively, upon detecting the start of spontaneous inhalation and exhalation respectively. RPT devices 4000 that “trigger” and “cycle” effectively perform discrete phase determination, since the trigger and cycle points are the instants at which the phase changes from exhalation to inhalation and from inhalation to exhalation, respectively. In one implementation of bi-valued phase determination, the phase output Φ is determined to have a discrete value of 0 (thereby “triggering” the RPT device 4000) when the respiratory flow rate Qr has a value that exceeds a positive threshold, and a discrete value of 0.5 revolutions (thereby “cycling” the RPT device 4000) when a respiratory flow rate Qr has a value that is more negative than a negative threshold. The inhalation time Ti and the exhalation time Te may be estimated as typical values over many respiratory cycles of the time spent with phase Φ equal to 0 (indicating inspiration) and 0.5 (indicating expiration) respectively.

Another implementation of discrete phase determination provides a tri-valued phase output Φ with a value of one of inhalation, mid-inspiratory pause, and exhalation.

In other forms, known as continuous phase determination, the phase output Φ is a continuous variable, for example varying from 0 to 1 revolutions, or 0 to 2π radians. RPT devices 4000 that perform continuous phase determination may trigger and cycle when the continuous phase reaches 0 and 0.5 revolutions, respectively. In one implementation of continuous phase determination, the phase Φ is first discretely estimated from the respiratory flow rate Qr as described above, as are the inhalation time Ti and the exhalation time Te. The continuous phase Φ at any given instant may be determined as the half the proportion of the inhalation time Ti that has elapsed since the previous trigger instant, or 0.5 revolutions plus half the proportion of the exhalation time Te that has elapsed since the previous cycle instant (whichever instant was more recent).

5.4.3.2.2 Waveform Determination

In one form of the present technology, the therapy parameter determination algorithm 4329 provides an approximately constant treatment pressure throughout a respiratory cycle of a patient.

In other forms of the present technology, the therapy control module 4330 controls the pressure generator 4140 to provide a treatment pressure Pt that varies as a function of phase Φ of a respiratory cycle of a patient according to a waveform template Π(Φ).

In one form of the present technology, a waveform determination algorithm 4322 provides a waveform template Π(Φ) with values in the range [0, 1] on the domain of phase values Φ provided by the phase determination algorithm 4321 to be used by the therapy parameter determination algorithm 4329.

In one form, suitable for either discrete or continuously-valued phase, the waveform template Π(Φ) is a square-wave template, having a value of 1 for values of phase up to and including 0.5 revolutions, and a value of 0 for values of phase above 0.5 revolutions. In one form, suitable for continuously-valued phase, the waveform template Π(Φ) comprises two smoothly curved portions, namely a smoothly curved (e.g. raised cosine) rise from 0 to 1 for values of phase up to 0.5 revolutions, and a smoothly curved (e.g. exponential) decay from 1 to 0 for values of phase above 0.5 revolutions. In one form, suitable for continuously-valued phase, the waveform template Π(Φ) is based on a square wave, but with a smooth rise from 0 to 1 for values of phase up to a “rise time” that is less than 0.5 revolutions, and a smooth fall from 1 to 0 for values of phase within a “fall time” after 0.5 revolutions, with a “fall time” that is less than 0.5 revolutions.

In some forms of the present technology, the waveform determination algorithm 4322 selects a waveform template Π(Φ) from a library of waveform templates, dependent on a setting of the RPT device. Each waveform template Π(Φ) in the library may be provided as a lookup table of values Π against phase values Φ. In other forms, the waveform determination algorithm 4322 computes a waveform template Π(Φ) “on the fly” using a predetermined functional form, possibly parametrised by one or more parameters (e.g. time constant of an exponentially curved portion). The parameters of the functional form may be predetermined or dependent on a current state of the patient 1000.

In some forms of the present technology, suitable for discrete bi-valued phase of either inhalation (Φ=0 revolutions) or exhalation (Φ=0.5 revolutions), the waveform determination algorithm 4322 computes a waveform template Π “on the fly” as a function of both discrete phase Φ and time t measured since the most recent trigger instant. In one such form, the waveform determination algorithm 4322 computes the waveform template Π(Φ, t) in two portions (inspiratory and expiratory) as follows:

${\Pi\left( {\Phi,t} \right)} = \left\{ \begin{matrix} {{\Pi_{i}(t)},} & {\Phi = 0} \\ {{\Pi_{e}\left( {t - T_{i}} \right)},} & {\Phi = 0.5} \end{matrix} \right.$

where Π_(i)(t) and Π_(e)(t) are inspiratory and expiratory portions of the waveform template Π(Φ, t). In one such form, the inspiratory portion Π_(i)(t) of the waveform template is a smooth rise from 0 to 1 parametrised by a rise time, and the expiratory portion Π_(e)(t) of the waveform template is a smooth fall from 1 to 0 parametrised by a fall time.

5.4.3.2.3 Tidal Volume Estimation

In some forms of the present technology, the central controller 4230 executes one or more tidal volume estimation algorithms 4323 for the estimation of tidal volume using the values returned by one or more of the other algorithms in the therapy engine module 4320.

In one form of the present technology, the tidal volume estimation algorithm 4323 receives as an input a signal indicative of respiratory flow rate Qr and the phase Φ determined by the phase determination algorithm 4321, and returns an estimate of the tidal volume V_(T) of the most recent breath. The tidal volume V_(T) may be estimated as the inspiratory (tidal) volume Vi for the breath, the expiratory (tidal) volume Ve for the breath, or some combination of the two, e.g. the mean or average. The inspiratory volume Vi may be estimated as the integral of the respiratory flow rate Qr over the inspiratory portion of the breath (indicated by phase Φ being less than 0.5). The expiratory volume Ve may be estimated as the integral of the respiratory flow rate Qr over the expiratory portion of the breath (indicated by phase Φ being greater than or equal to 0.5).

5.4.3.2.4 Determination of Therapy Parameters

In some forms of the present technology, the central controller 4230 executes one or more therapy parameter determination algorithms 4329 for the determination of one or more therapy parameters using the values returned by one or more of the other algorithms in the therapy engine module 4320.

In one form of the present technology, the therapy parameter is an instantaneous treatment pressure Pt. In one implementation of this form, the therapy parameter determination algorithm 4329 determines the treatment pressure Pt using the equation Pt=AΠ(Φ,t)+P ₀  (1)

where:

-   -   A is the amplitude,     -   Π(Φ, t) is the waveform template value (in the range 0 to 1) at         the current value Φ of phase and t of time, and     -   P₀ is a base pressure.

If the waveform determination algorithm 4322 provides the waveform template Π(Φ, t) as a lookup table of values Π indexed by phase Φ, the therapy parameter determination algorithm 4329 applies equation (1) by locating the nearest lookup table entry to the current value Φ of phase returned by the phase determination algorithm 4321, or by interpolation between the two entries straddling the current value Φ of phase.

The values of the amplitude A and the base pressure P₀ may be set by the therapy parameter determination algorithm 4329 depending on the chosen respiratory pressure therapy mode in the manner described below.

5.4.3.3 Therapy Control Module

The therapy control module 4330 in accordance with one aspect of the present technology receives as inputs the therapy parameters from the therapy parameter determination algorithm 4329 of the therapy engine module 4320, and controls the pressure generator 4140 to deliver a flow of air in accordance with the therapy parameters.

In one form of the present technology, the therapy parameter is a treatment pressure Pt, and the therapy control module 4330 controls the pressure generator 4140 to deliver a flow of air whose mask pressure Pm at the patient interface 3000 is equal to the treatment pressure Pt.

5.4.3.4 Detection of Fault Conditions

In one form of the present technology, the central controller 4230 executes one or more methods 4340 for the detection of fault conditions. The fault conditions detected by the one or more methods 4340 may include at least one of the following:

-   -   Power failure (no power, or insufficient power)     -   Transducer fault detection     -   Failure to detect the presence of a component     -   Operating parameters outside recommended ranges (e.g. pressure,         flow rate, temperature, PaO₂)     -   Failure of a test alarm to generate a detectable alarm signal.

Upon detection of the fault condition, the corresponding algorithm 4340 signals the presence of the fault by one or more of the following:

-   -   Initiation of an audible, visual &/or kinetic (e.g. vibrating)         alarm     -   Sending a message to an external device     -   Logging of the incident

5.5 Air Circuit

An air circuit 4170 in accordance with an aspect of the present technology is a conduit or a tube constructed and arranged to allow, in use, a flow of air to travel between two components such as the RPT device 4000 and the patient interface 3000.

In particular, the air circuit 4170 may be in fluid connection with the outlet of the pneumatic block 4020 and the patient interface. The air circuit may be referred to as an air delivery tube. In some cases, there may be separate limbs of the circuit for inhalation and exhalation. In other cases, a single limb air circuit is used.

5.6 Humidifier

In one form of the present technology there is provided a humidifier 5000 (e.g. as shown in FIG. 5A) to change the absolute humidity of air or gas for delivery to a patient relative to ambient air. Typically, the humidifier 5000 is used to increase the absolute humidity and increase the temperature of the flow of air (relative to ambient air) before delivery to the patient's airways.

The humidifier 5000 may comprise a humidifier reservoir 5110, a humidifier inlet 5002 to receive a flow of air, and a humidifier outlet 5004 to deliver a humidified flow of air. In some forms, as shown in FIG. 5A and FIG. 5B, an inlet and an outlet of the humidifier reservoir 5110 may be the humidifier inlet 5002 and the humidifier outlet 5004 respectively. The humidifier 5000 may further comprise a humidifier base 5006, which may be adapted to receive the humidifier reservoir 5110 and comprise a heating element 5240.

5.7 Breathing Waveforms

FIG. 6 shows a model typical breath waveform of a person while sleeping. The horizontal axis is time, and the vertical axis is respiratory flow rate. While the parameter values may vary, a typical breath may have the following approximate values: tidal volume V_(T) 0.5 L, inhalation time Ti 1.6 s, peak inspiratory flow rate Qpeak 0.4 L/s, exhalation time Te 2.4 s, peak expiratory flow rate Qpeak −0.5 L/s. The total duration of the breath Ttot is about 4 s. The person typically breathes at a rate of about 15 breaths per minute (BPM), with Ventilation Vent about 7.5 L/min. A typical duty cycle, the ratio of Ti to Ttot, is about 40%.

5.8 Respiratory Pressure Therapy Modes

Various respiratory pressure therapy modes may be implemented by the RPT device 4000 depending on the values of the parameters A and P₀ in the treatment pressure equation (1) used by the therapy parameter determination algorithm 4329 in one form of the present technology.

In some implementations of this form of the present technology, the amplitude A in the treatment pressure equation (1) is identically zero, so the treatment pressure Pt is identically equal to the base pressure P₀ throughout the respiratory cycle. Such implementations are generally grouped under the heading of CPAP therapy. In other implementations of this form of the present technology, the value of amplitude A in equation (1) is positive. Such implementations are known as bi-level therapy, because in determining the treatment pressure Pt using equation (1) with positive amplitude A, the therapy parameter determination algorithm 4329 oscillates the treatment pressure Pt between two values or levels in synchrony with the spontaneous respiratory effort of the patient 1000. That is, based on the typical waveform templates MD, t) described above, the therapy parameter determination algorithm 4329 increases the treatment pressure Pt to P₀+A (known as the IPAP) at the start of, or during, or inspiration and decreases the treatment pressure Pt to the base pressure P₀ (known as the EPAP) at the start of, or during, expiration.

In some forms of bi-level therapy, the amplitude A is large enough that the RPT device 4000 does some or all of the work of breathing of the patient 1000. Such forms may be referred to as providing ventilatory support to the patient 1000. In such forms, known as pressure support or pressure controlled ventilation therapy, the amplitude A is referred to as the pressure support, or swing. In pressure support ventilation therapy, the IPAP is the base pressure P₀ plus the pressure support A, and the EPAP is the base pressure P₀.

In some forms of pressure support ventilation therapy, known as fixed pressure support ventilation therapy, the pressure support A is fixed at a predetermined value, e.g. 10 cmH₂O. The predetermined pressure support value is a setting of the RPT device 4000, and may be set for example by hard-coding during configuration of the RPT device 4000 or by manual entry through the input device 4220.

In other forms of pressure support ventilation therapy, broadly known as servo-ventilation, the therapy parameter determination algorithm 4329 takes as input some currently measured or estimated parameter of the respiratory cycle and a target value of that respiratory parameter, and continuously adjusts the parameters of equation (1) to adjust the current measure of the respiratory parameter towards the target value. One example of servo-ventilation in which the respiratory parameter is tidal volume V_(T) is sometimes referred to as safety volume mode.

In some forms of servo-ventilation, the therapy parameter determination algorithm 4329 applies a servo-control methodology to repeatedly compute the pressure support A so as to adjust the current measure of the respiratory parameter towards the target value. One such servo-control methodology is Proportional-Integral (PI) control. In one implementation of PI control for safety volume mode, an adjustment ΔA to the current pressure support A is computed as:

$\begin{matrix} {{\Delta\; A} = {\frac{G}{C_{nom}}\left( {{V_{T}({target})} - V_{T}} \right)}} & (2) \end{matrix}$

where G is the servo-control gain, Cnom is a nominal compliance constant (which is typically set to 60 ml/cmH₂O for adults and to 40 ml/cmH₂O for pediatrics, but may be varied for different patient sub-types) and V_(T)(target) is the target tidal volume (in millilitres). The gain G is typically a constant value of one.

Other servo-control methodologies that may be applied by the therapy parameter determination algorithm 4329 include proportional (P), proportional-differential (PD), and proportional-integral-differential (PID).

The value of the pressure support A computed via equation (2) may be clipped to a range defined as [Amin, Amax]. In this implementation, the pressure support A sits by default at the minimum pressure support Amin until the measure of tidal volume V_(T) falls below the target tidal volume V_(T)(target), at which point A starts increasing, only falling back to Amin when V_(T) exceeds V_(T)(target) once again.

The pressure support limits Amin and Amax are settings of the RPT device 4000, set for example by hard-coding during configuration of the RPT device 4000 or by manual entry through the input device 4220.

5.8.1 Sudden Leak Change Handling

Sudden change in leak (either the sudden appearance of a leak or its sudden resolution) may cause the estimate of leak flow rate Ql, and hence the respiratory flow rate estimate Qr, to be temporarily inaccurate while the leak flow rate estimation algorithm 4316 “catches up” with the sudden change. When the leak appears, the respiratory flow rate Qr is overestimated for a period lasting perhaps a few breaths. Then, when the leak is resolved, the respiratory flow rate Qr is underestimated for a period. In addition, the phase determination algorithm 4321 may be caused to overestimate or underestimate the inspiratory portions of the breaths respectively. The result is that when the leak appears, the inspiratory volume Vi tends to be temporarily overestimated and the expiratory volume Ve tends to be temporarily underestimated. When the leak is resolved, the expiratory volume Ve tends to be temporarily overestimated and the inspiratory volume Vi tends to be temporarily underestimated. In either case, the estimate of tidal volume V_(T) by the tidal volume estimation algorithm 4323 tends to be temporarily higher than the actual tidal volume. The result in safety volume mode is that pressure support A is either inappropriately reduced, or increased more slowly than it should be. FIG. 7A contains a graph 7100 illustrating an example of such behaviour. The traces 7110 to 7130 in the graph 7100 represent direct measurements of various breathing parameters, rather than those parameters as estimated by the RPT device 4000 using the algorithms 4300. The top trace 7110 represents the treatment pressure Pt, oscillating with amplitude equal to the pressure support A. The middle trace 7120 represents the respiratory flow rate Qr, oscillating between positive (inspiration) and negative (expiration) portions. The lower trace 7130 represents the integral of respiratory flow rate Qr, i.e. the instantaneous volume. The peak value of the peaks in the trace 7130 therefore represents the tidal volume V_(T) of each successive breath.

A leak suddenly appears at 7140. The pressure support falls significantly soon afterwards as the tidal volume is overestimated, before recovering to its previous value after a dozen or so breaths. The result is a temporary fall in the delivered tidal volume V_(T). The leak is resolved at 7150, and once again the pressure support falls significantly as the tidal volume is once again overestimated, taking many breaths to recover its previous value, while the delivered tidal volume V_(T) also falls significantly.

In one form of the present technology, the servo-control gain G in equation (2) may be adjusted dynamically so that this effect of a sudden change in leak is reduced. As mentioned above, one effect of sudden leak appearance or resolution is to cause the estimates of inspiratory volume Vi and expiratory volume Ve to diverge temporarily. In one implementation, the servo-control gain G is therefore adjusted based on a difference between the estimates of inspiratory volume Vi and expiratory volume Ve, so as to generally decrease as the difference increases. In one such implementation, the servo-control gain G is adjusted linearly between its default value and a lower value Gmin as a relative differential tidal volume dv varies between a low value dvmin and a default value, e.g. 1. FIG. 8 illustrates one example of such an adjustment of the servo-control gain G, where the default value is one, the low value Gmin is 0.2, and the low value dvmin is also 0.2. The relative differential tidal volume dv may be computed from the most recent breath as

$\begin{matrix} {{dv} = \frac{{{Vi} - {Ve}}}{V_{T}}} & (3) \end{matrix}$

with adjustments to ensure the denominator is never zero.

By this implementation, the rate of adjustment of pressure support A is reduced as a sudden leak appearance or resolution causes the estimates of inspiratory volume Vi and expiratory volume Ve to diverge temporarily. Once these estimates converge as the leak flow rate estimation algorithm 4316 catches up with the sudden leak change, the servo-control gain G returns to its normal value of one. The effects of the sudden leak change on the servo-control of pressure support are thus smoothed out, giving a more stable therapy in the face of sudden changes in leak.

FIG. 7B contains a graph 7200 illustrating of the behaviour of the therapy parameter determination algorithm 4329 in one such implementation of the present technology. As in FIG. 7A, the traces 7210 to 7230 in the graph 7200 represent direct measurements of various breathing parameters, rather than those parameters as estimated by the RPT device 4000 using the algorithms 4300. The top trace 7210 represents the treatment pressure Pt, the middle trace 7220 represents the respiratory flow rate Qr, and the lower trace 7230 represents the integral of respiratory flow rate Qr, i.e. the instantaneous volume.

A leak suddenly appears at 7240, but the pressure support A falls far less than in the trace 7110 at the same stage. As a result, the delivered tidal volume also falls by less than in the trace 7130 at the same stage. The leak is resolved at 7250, and once again the pressure support A falls far less than in the trace 7110, resulting in a smaller fall in the delivered tidal volume than in the trace 7130 at the same stage. The delivered tidal volume V_(T) therefore varies significantly less in the face of the sudden leak changes.

FIG. 9 shows a high level flow chart of a method 9000 of dynamically adjusting the servo-control gain G in the form of an algorithm that may be implemented in, or as a separate module of, a therapy engine module 4320 in a respiratory therapy apparatus such as RPT 4000. At block 9004, the algorithm is invoked upon detection of a sudden change in leak. As shown in FIG. 7A, sudden leak detection may be made dependent on a change in pressure support or tidal volume. A sudden leak may be determined based on a rise in the relative differential tidal volume dv (see equation (3)) between the estimates of inspiratory volume Vi and expiratory volume Ve above some threshold dvmin over some predetermined time period (e.g., a single or multiple breaths). Upon detection of the sudden leak, the servo-control gain G of the apparatus is adjusted at block 9008 based on a difference between estimated volume values Vi and Ve. As shown in FIG. 8 and as defined in equation (3), the adjustment may vary linearly between a default or normal upper value and a lower minimum value, Gmin based on a relative differential tidal volume dv. The gain G is thus adjusted dynamically in response to sudden leak which improves the performance of apparatus incorporating the technology by servo-controlling the degree of support in response to the sudden leak. Thus, the effect of the leak is reduced and therefore the impact of the leak on the operation in safety volume mode is also reduced and may be reflected in the change of ventilatory support provided.

At decision diamond 9012, the difference between Vi and Ve is examined to determine whether there has been convergence, e.g., the difference between Vi and Ve has converged within some predetermined limit such as relative difference of 20% or less. If there is convergence, the gain G is then returned to its normal or default value (e.g. 1) at block 9016. If there has been no convergence, then the gain G may be further adjusted at block 9008 depending on the relative difference between the estimated inspiratory and expiratory volumes Vi, Ve. This allows for an additional level of dynamic adjustment. For example, until the convergence criterion is met, the gain G may be adjusted linearly based on the computed relative differential tidal volume dv as illustrated in FIG. 8.

In an alternative implementation, blocks 9004, 9012, and 9016 are not used, and block 9008 is repeatedly invoked to adjust the servo-control gain G of the apparatus based on a difference between estimated volume values Vi and Ve.

As an example, a respiratory therapy apparatus that includes a pressure generator, transducer and controller may be configured to operate in accordance with an aspect of the present technology. In this example, the pressure generator would generate a flow of air or gas useful in providing ventilatory support to a patient. The transducer may they then generate a signal that represents one or more properties of the flow of air. A controller would then process or analyse the signal to estimate values for inspiratory and expiratory volumes of one or more patient breaths. The controller would also be configured to servo-control the ventilatory support to adjust an estimated tidal volume toward a target volume using a gain that dynamically adjusts based on a difference between the estimated values for inspiratory and expiratory volumes. This results in improvement relating to the adjustment of the estimated tidal volume in the presence of sudden leaks.

5.9 Glossary

For the purposes of the present technology disclosure, in certain forms of the present technology, one or more of the following definitions may apply. In other forms of the present technology, alternative definitions may apply.

5.9.1 General

Air: In certain forms of the present technology, air may be taken to mean atmospheric air, and in other forms of the present technology air may be taken to mean some other combination of breathable gases, e.g. atmospheric air enriched with oxygen.

Ambient: In certain forms of the present technology, the term ambient will be taken to mean (i) external of the treatment system or patient, and (ii) immediately surrounding the treatment system or patient.

For example, ambient humidity with respect to a humidifier may be the humidity of air immediately surrounding the humidifier, e.g. the humidity in the room where a patient is sleeping. Such ambient humidity may be different to the humidity outside the room where a patient is sleeping.

In another example, ambient pressure may be the pressure immediately surrounding or external to the body.

In certain forms, ambient (e.g., acoustic) noise may be considered to be the background noise level in the room where a patient is located, other than for example, noise generated by an RPT device or emanating from a mask or patient interface. Ambient noise may be generated by sources outside the room.

Automatic Positive Airway Pressure (APAP) therapy: CPAP therapy in which the treatment pressure is automatically adjustable, e.g. from breath to breath, between minimum and maximum limits, depending on the presence or absence of indications of SDB events.

Continuous Positive Airway Pressure (CPAP) therapy: Respiratory pressure therapy in which the treatment pressure is approximately constant through a respiratory cycle of a patient. In some forms, the pressure at the entrance to the airways will be slightly higher during exhalation, and slightly lower during inhalation. In some forms, the pressure will vary between different respiratory cycles of the patient, for example, being increased in response to detection of indications of partial upper airway obstruction, and decreased in the absence of indications of partial upper airway obstruction.

Flow rate: The volume (or mass) of air delivered per unit time. Flow rate may refer to an instantaneous quantity. In some cases, a reference to flow rate will be a reference to a scalar quantity, namely a quantity having magnitude only. In other cases, a reference to flow rate will be a reference to a vector quantity, namely a quantity having both magnitude and direction. Flow rate may be given the symbol Q. ‘Flow rate’ is sometimes shortened to simply ‘flow’ or ‘airflow’.

In the example of patient respiration, a flow rate may be nominally positive for the inspiratory portion of a breathing cycle of a patient, and hence negative for the expiratory portion of the breathing cycle of a patient. Total flow rate, Qt, is the flow rate of air leaving the RPT device. Vent flow rate, Qv, is the flow rate of air leaving a vent to allow washout of exhaled gases. Leak flow rate, Ql, is the flow rate of leak from a patient interface system or elsewhere. Respiratory flow rate, Qr, is the flow rate of air that is received into the patient's respiratory system.

Humidifier: The word humidifier will be taken to mean a humidifying apparatus constructed and arranged, or configured with a physical structure to be capable of providing a therapeutically beneficial amount of water (H₂O) vapour to a flow of air to ameliorate a medical respiratory condition of a patient.

Leak: The word leak will be taken to be a flow of air to or from ambient other than through the elements of the air circuit. In one example, leak may occur as the result of an incomplete seal between a mask and a patient's face. In another example leak may occur in a swivel elbow to the ambient. Leak may also encompass the air exhaled to ambient around a tracheostomy tube in invasive ventilation.

Noise, conducted (acoustic): Conducted noise in the present document refers to noise which is carried to the patient by the pneumatic path, such as the air circuit and the patient interface as well as the air therein. In one form, conducted noise may be quantified by measuring sound pressure levels at the end of an air circuit.

Noise, radiated (acoustic): Radiated noise in the present document refers to noise which is carried to the patient by the ambient air. In one form, radiated noise may be quantified by measuring sound power/pressure levels of the object in question according to ISO 3744.

Noise, vent (acoustic): Vent noise in the present document refers to noise which is generated by the flow of air through any vents such as vent holes of the patient interface.

Patient: A person, whether or not they are suffering from a respiratory condition.

Pressure: Force per unit area. Pressure may be expressed in a range of units, including cmH₂O, g-f/cm² and hectopascal. 1 cmH₂O is equal to 1 g-f/cm² and is approximately 0.98 hectopascal. In this specification, unless otherwise stated, pressure is given in units of cmH₂O.

The pressure in the patient interface is given the symbol Pm, while the treatment pressure, which represents a target value to be achieved by the mask pressure Pm at the current instant of time, is given the symbol Pt.

Respiratory Pressure Therapy (RPT): The application of a supply of air to an entrance to the airways at a treatment pressure that is typically positive with respect to atmosphere.

Ventilator: A mechanical device that provides pressure support to a patient to perform some or all of the work of breathing.

5.9.2 Respiratory Cycle

Apnea: According to some definitions, an apnea is said to have occurred when flow falls below a predetermined threshold for a duration, e.g. 10 seconds. An obstructive apnea will be said to have occurred when, despite patient effort, some obstruction of the airway does not allow air to flow. A central apnea will be said to have occurred when an apnea is detected that is due to a reduction in breathing effort, or the absence of breathing effort, despite the airway being patent. A mixed apnea occurs when a reduction or absence of breathing effort coincides with an obstructed airway.

Breathing rate: The rate of spontaneous respiration of a patient, usually measured in breaths per minute.

Duty cycle: The ratio of inhalation time, Ti to total breath time, Ttot.

Effort (breathing): The work done by a spontaneously breathing person attempting to breathe.

Expiratory portion of a breathing cycle: The period from the start of expiratory flow to the start of inspiratory flow.

Flow limitation: Flow limitation will be taken to be the state of affairs in a patient's respiration where an increase in effort by the patient does not give rise to a corresponding increase in flow. Where flow limitation occurs during an inspiratory portion of the breathing cycle it may be described as inspiratory flow limitation. Where flow limitation occurs during an expiratory portion of the breathing cycle it may be described as expiratory flow limitation.

Hypopnea: According to some definitions, a hypopnea is taken to be a reduction in flow, but not a cessation of flow. In one form, a hypopnea may be said to have occurred when there is a reduction in flow below a threshold rate for a duration. A central hypopnea will be said to have occurred when a hypopnea is detected that is due to a reduction in breathing effort. In one form in adults, either of the following may be regarded as being hypopneas:

-   -   (i) a 30% reduction in patient breathing for at least 10 seconds         plus an associated 4% desaturation; or     -   (ii) a reduction in patient breathing (but less than 50%) for at         least 10 seconds, with an associated desaturation of at least 3%         or an arousal.

Hyperpnea: An increase in flow to a level higher than normal.

Inspiratory portion of a breathing cycle: The period from the start of inspiratory flow to the start of expiratory flow will be taken to be the inspiratory portion of a breathing cycle.

Patency (airway): The degree of the airway being open, or the extent to which the airway is open. A patent airway is open. Airway patency may be quantified, for example with a value of one (1) being patent, and a value of zero (0), being closed (obstructed).

Positive End-Expiratory Pressure (PEEP): The pressure above atmosphere in the lungs that exists at the end of expiration.

Peak flow rate (Qpeak): The maximum value of flow rate during the inspiratory portion of the respiratory flow waveform.

Respiratory flow rate, patient airflow rate, respiratory airflow rate (Qr): These terms may be understood to refer to the RPT device's estimate of respiratory flow rate, as opposed to “true respiratory flow rate” or “true respiratory flow rate”, which is the actual respiratory flow rate experienced by the patient, usually expressed in litres per minute.

Tidal volume (V_(T)): The volume of air inhaled or exhaled during normal breathing, when extra effort is not applied. In principle the inspiratory volume Vi (the volume of air inhaled) is equal to the expiratory volume Ve (the volume of air exhaled), and therefore a single tidal volume V_(T) may be defined as equal to either quantity. In practice the tidal volume V_(T) is estimated as some combination, e.g. the mean, of these two quantities.

(inhalation) Time (Ti): The duration of the inspiratory portion of the respiratory flow rate waveform.

(exhalation) Time (Te): The duration of the expiratory portion of the respiratory flow rate waveform.

(total) Time (Ttot): The total duration between the start of one inspiratory portion of a respiratory flow rate waveform and the start of the following inspiratory portion of the respiratory flow rate waveform.

Typical recent ventilation: The value of ventilation around which recent values of ventilation Vent over some predetermined timescale tend to cluster, that is, a measure of the central tendency of the recent values of ventilation.

Upper airway obstruction (UAO): includes both partial and total upper airway obstruction. This may be associated with a state of flow limitation, in which the flow rate increases only slightly or may even decrease as the pressure difference across the upper airway increases (Starling resistor behaviour).

Ventilation (Vent): A measure of a rate of gas being exchanged by the patient's respiratory system. Measures of ventilation may include one or both of inspiratory and expiratory flow, per unit time. When expressed as a volume per minute, this quantity is often referred to as “minute ventilation”. Minute ventilation is sometimes given simply as a volume, understood to be the volume per minute.

5.9.3 Ventilation

Adaptive Servo-Ventilator (ASV): A servo-ventilator that has a changeable, rather than fixed target property. The changeable target property may be learned from some characteristic of the patient, for example, a respiratory characteristic of the patient.

Backup rate: A parameter of a ventilator that establishes the minimum breathing rate (typically in number of breaths per minute) that the ventilator will deliver to the patient, if not triggered by spontaneous respiratory effort.

Cycled: The termination of a ventilator's inspiratory phase. When a ventilator delivers a breath to a spontaneously breathing patient, at the end of the inspiratory portion of the breathing cycle, the ventilator is said to be cycled to stop delivering the breath.

Expiratory positive airway pressure (EPAP): a base pressure, to which a pressure varying within the breath is added to produce the desired mask pressure which the ventilator will attempt to achieve at a given time.

End expiratory pressure (EEP): Desired mask pressure which the ventilator will attempt to achieve at the end of the expiratory portion of the breath. If the pressure waveform template Π(Φ) is zero-valued at the end of expiration, i.e. Π(Φ)=0 when Φ=1, the EEP is equal to the EPAP.

Inspiratory positive airway pressure (IPAP): Maximum desired mask pressure which the ventilator will attempt to achieve during the inspiratory portion of the breath.

Pressure support: A number that is indicative of the increase in pressure during ventilator inspiration over that during ventilator expiration, and generally means the difference in pressure between the maximum value during inspiration and the base pressure (e.g., PS=IPAP−EPAP). In some contexts pressure support means the difference which the ventilator aims to achieve, rather than what it actually achieves.

Servo-ventilator: A ventilator that measures or estimates some parameter of the patient's respiratory cycle and adjusts the level of pressure support to adjust the measured parameter towards a target value of the parameter.

Spontaneous/Timed (S/T): A mode of a ventilator or other device that attempts to detect the initiation of a breath of a spontaneously breathing patient. If however, the device is unable to detect a breath within a predetermined period of time, the device will automatically initiate delivery of the breath.

Swing: Equivalent term to pressure support.

Triggered: When a ventilator delivers a breath of air to a spontaneously breathing patient, it is said to be triggered to do so at the initiation of the respiratory portion of the breathing cycle by the patient's efforts.

5.10 Other Remarks

A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in Patent Office patent files or records, but otherwise reserves all copyright rights whatsoever.

Unless the context clearly dictates otherwise and where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit, between the upper and lower limit of that range, and any other stated or intervening value in that stated range is encompassed within the technology. The upper and lower limits of these intervening ranges, which may be independently included in the intervening ranges, are also encompassed within the technology, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the technology.

Furthermore, where a value or values are stated herein as being implemented as part of the technology, it is understood that such values may be approximated, unless otherwise stated, and such values may be utilized to any suitable significant digit to the extent that a practical technical implementation may permit or require it.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this technology belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present technology, a limited number of the exemplary methods and materials are described herein.

When a particular material is identified as being used to construct a component, obvious alternative materials with similar properties may be used as a substitute. Furthermore, unless specified to the contrary, any and all components herein described are understood to be capable of being manufactured and, as such, may be manufactured together or separately.

It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include their plural equivalents, unless the context clearly dictates otherwise.

All publications mentioned herein are incorporated herein by reference in their entirety to disclose and describe the methods and/or materials which are the subject of those publications. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present technology is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates, which may need to be independently confirmed.

The terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.

The subject headings used in the detailed description are included only for the ease of reference of the reader and should not be used to limit the subject matter found throughout the disclosure or the claims. The subject headings should not be used in construing the scope of the claims or the claim limitations.

Although the technology herein has been described with reference to particular examples, it is to be understood that these examples are merely illustrative of the principles and applications of the technology. In some instances, the terminology and symbols may imply specific details that are not required to practice the technology. For example, although the terms “first” and “second” may be used, unless otherwise specified, they are not intended to indicate any order but may be utilised to distinguish between distinct elements. Furthermore, although process steps in the methodologies may be described or illustrated in an order, such an ordering is not required. Those skilled in the art will recognize that such ordering may be modified and/or aspects thereof may be conducted concurrently or even synchronously.

It is therefore to be understood that numerous modifications may be made to the illustrative examples and that other arrangements may be devised without departing from the spirit and scope of the technology.

5.11 Reference Signs List

-   -   patient 1000     -   patient interface 3000     -   seal-forming structure 3100     -   plenum chamber 3200     -   structure 3300     -   vent 3400     -   connection port 3600     -   forehead support 3700     -   RPT device 4000     -   external housing 4010     -   upper portion 4012     -   portion 4014     -   panel s 4015     -   chassis 4016     -   handle 4018     -   pneumatic block 4020     -   air filters 4110     -   inlet air filter 4112     -   outlet air filter 4114     -   mufflers 4120     -   inlet muffler 4122     -   outlet muffler 4124     -   pressure generator 4140     -   blower 4142     -   motor 4144     -   anti-spill back valve 4160     -   air circuit 4170     -   electrical components 4200     -   PCBA 4202     -   power supply 4210     -   input devices 4220     -   central controller 4230     -   clock 4232     -   therapy device controller 4240     -   protection circuits 4250     -   memory 4260     -   transducers 4270     -   pressure sensor 4272     -   flow rate sensors 4274     -   motor speed transducer 4276     -   data communication interface 4280     -   remote external communication network 4282     -   local external communication network 4284     -   remote external device 4286     -   local external device 4288     -   output device 4290     -   display driver 4292     -   display 4294     -   algorithms 4300     -   pre-processing module 4310     -   pressure compensation algorithm 4312     -   vent flow rate estimation algorithm 4314     -   leak flow rate estimation algorithm 4316     -   respiratory flow rate estimation algorithm 4318     -   therapy engine module 4320     -   phase determination algorithm 4321     -   waveform determination algorithm 4322     -   tidal volume estimation algorithm 4323     -   therapy parameter determination algorithm 4329     -   therapy control module 4330     -   fault condition detection methods 4340     -   humidifier 5000     -   humidifier inlet 5002     -   humidifier outlet 5004     -   humidifier base 5006     -   humidifier reservoir 5110     -   humidifier reservoir dock 5130     -   heating element 5240     -   graph 7100     -   trace 7110     -   trace 7120     -   trace 7130     -   graph 7200     -   trace 7210     -   trace 7220     -   trace 7230     -   sudden leak change 7240     -   leak resolution 7250     -   method 9000     -   block 9004     -   block 9008     -   decision diamond 9012     -   block 9016 

The invention claimed is:
 1. Apparatus for treating a respiratory disorder in a patient, the apparatus comprising: a pressure generator configured to generate a flow of air so as to provide ventilatory support to the patient; a transducer configured to generate a signal representing a property of the flow of air; and a controller configured to: analyse the signal to estimate an inspiratory volume and an expiratory volume of a breath of the patient; and servo-control a degree of the ventilatory support to adjust an estimated tidal volume toward a target tidal volume, wherein a gain of the servo-control is dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume.
 2. The apparatus of claim 1, wherein the gain decreases as the difference between the estimated inspiratory volume and the estimated expiratory volume increases.
 3. The apparatus of claim 1, wherein the gain is dependent on the difference between the estimated inspiratory volume and the estimated expiratory volume relative to the estimated tidal volume.
 4. The apparatus of claim 1, wherein the gain is dependent on the absolute magnitude of the difference between the estimated inspiratory volume and the estimated expiratory volume.
 5. The apparatus of claim 1, wherein the degree of the ventilatory support is a pressure support of the ventilatory support.
 6. A method of operating a respiratory treatment apparatus configured to generate a flow of air so as to provide ventilatory support to a patient, the method comprising: measuring a property of the flow of air, using a transducer; analysing, in a controller, the measured property to estimate an inspiratory volume and an expiratory volume of a breath of the patient; calculating, in a controller, a gain dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume; and servo-controlling, by a controller, a degree of the ventilatory support using the calculated gain to adjust an estimated tidal volume for the patient toward a target tidal volume.
 7. The method of claim 6, wherein the calculating decreases the gain as the difference between the estimated inspiratory volume and the estimated expiratory volume increases.
 8. The method of claim 6, wherein the gain is dependent on the difference between the estimated inspiratory volume and the estimated expiratory volume relative to the estimated tidal volume.
 9. The method of claim 6, wherein the gain is dependent on the absolute magnitude of the difference between the estimated inspiratory volume and the estimated expiratory volume.
 10. The method of claim 6, wherein the degree of the ventilatory support is a pressure support of the ventilatory support.
 11. A system for treating a respiratory disorder in a patient, comprising: means for generating a flow of air so as to provide ventilatory support to the patient; means for generating a signal representing a property of the flow of air; means for analysing the signal to estimate an inspiratory volume and an expiratory volume of a breath of the patient; and means for servo-controlling a degree of the ventilatory support to adjust an estimated tidal volume toward a target tidal volume, wherein a gain of the servo-control is dependent on a difference between the estimated inspiratory volume and the estimated expiratory volume.
 12. Apparatus for treating a respiratory disorder in a patient, the apparatus comprising: a blower configured to deliver a supply of air that provides ventilatory support to the patient; a transducer configured to generate a signal representing a property of the supply of air; and a controller configured to: analyse the signal to estimate an inspiratory volume and an expiratory volume of a breath of the patient; and adjust servo-control gain based on a difference between the estimated inspiratory volume and the estimated expiratory volume.
 13. The apparatus of claim 12, wherein the controller adjusts the servo-control gain so that the gain decreases as the difference between the estimated inspiratory volume and the estimated expiratory volume increases.
 14. The apparatus of claim 12, wherein an adjustment in servo-control gain causes a reduction in a rate of adjustment of pressure support of the supply of air.
 15. The apparatus of claim 12, wherein the controller is configured to adjust the servo-control gain when the difference is greater than or equal to 20%.
 16. The apparatus of claim 12, wherein the controller is further configured to adjust the servo-control gain to a default value when the difference returns to a value that is less than 20%. 